When beams of high energy x-rays or electrons are used for radiotherapy, it is important to direct the beams to a tumor within the patient, while restricting the beams from striking healthy tissue outside the tumor region. Tumors commonly have irregular shapes and it is necessary to shape the beam cross-section to the corresponding irregular shape. It is common for the treatment plan to prescribe the beam to be directed at the tumor from a number of different angles, where the beam profile is unique for each corresponding angle.
Currently, radiotherapy accelerators producing therapeutic electron beams utilize “applicators”, also known as “cones”, are attached to the therapy machine to provide a final collimation aperture along the beam path before the tumor is exposed. The applicator defines the final beam cross-section profile and it is desirable to place the applicator as close to the patient as possible to limit exposure to healthy tissue. Because the tumor has a unique shape for each prescribed exposure angle, a unique collimating aperture is required for each corresponding angle. It is common to have multiple apertures fabricated for treating a single tumor, where alloys with low melting temperatures are typically cast into the required irregular shapes. The cast apertures can be interchanged with the radiotherapy device to provide a beam that conforms to the shape of the area to be irradiated. These unique apertures are expensive and time consuming to fabricate.
In an attempt to alleviate the need to fabricate a unique aperture for each exposure, multi-leaf collimators (MLC) have been implemented as a way to shape the beam cross-section. These devices include a set of flat, thin leaves made from a high-density material, such as tungsten, where each leaf in moved transversely in and out of the radiation field to selectively attenuate portions of the beam to create a unique beam cross-section. The shape of the beam can be altered dynamically during the therapy session using motorized controls connected to each leaf. By dynamically attenuating select portions of the beam, intensity-modulated radiotherapy (IMRT) has been made possible, where by moving the leaves during beam exposure, the beam can be delivered in a manner such that the spatial fluence of the irradiation is not constant over the irradiated area. IMRT can also be accomplished by making multiple irradiations, each with a different field shape, the sum of which creates a field of non-uniform intensity. The leaves must be thick enough to highly attenuate the beam. For example, when using x-ray beams, at least a 6 cm thickness of tungsten is required.
X-ray MLC's are typically mounted as far from the patient as practicable to ensure maximum clearance between the radiotherapy machine and the patient. In accordance with some accelerators, the MLC has been used to replace the standard field-shaping jaws of the accelerator. The shape of the portion of the leaf that defines the edge of the field is designed for minimum penumbra to create the sharpest edge of the beam as possible between the irradiated and protected areas.
In electron radiotherapy, fabricated electron applicators are typically used, where the applicators are customized for each patient to define each unique final beam aperture. This process is very time consuming and expensive. The custom aperture must be installed by hand for each treatment field. If two or more fields are used for a therapy session, the aperture must be changed for each field. Further, the aperture must be redesigned to accommodate changes in the tumor size during the course of treatment. The beneficial practice of IMRT delivery cannot be used with these fixed apertures.
It is desirable to be able to use multi-leaf collimators for electron irradiation as well as for x-ray irradiation. Currently, MLC's that are designed for x-rays are not suitable to this end. To produce a desired penumbra, a collimator for electron beams must be close to the patient surface, typically within 5 cm. Conversely, the usual location of an x-ray MLC is far from the patient, which makes creating desirable beam characteristics unfeasible. It is possible to move the patient closer, but the penumbra achievable still cannot match that which is attained with an electron applicator.
Attempts to create the final aperture of an electron applicator using a form of MLC have been reported. In these efforts, the final aperture of the applicator has been constructed of a bank of leaves that can be moved to a variable position relative to the beam, similar to an x-ray MLC. The leaves do not have to be as thick as those for x-ray MLC's, where it requires only approximately 1 cm of brass to stop 20 MeV electrons compared to the 6 cm of more of tungsten required for an effective x-ray MLC.
FIG. 1 shows a perspective view of a prior art applicator and multi-leaf collimator assembly 100 for use with electron beams 101, where shown is an applicator 102 and a multi-leaf collimator 104 having two opposing sets of movable leaves 106 configured to move parallel with one another in a collimator housing 108 disposed as a treatment aperture 110. As shown, the leaves 108 are positioned manually however motorized leaves are also known in the art.
FIG. 2 is a planar view of a treatment configuration 200, where a patient 202 is positioned close to the prior art multi-leaf collimator assembly 100 attached to an accelerator 204. Here, the large extensions of the MLC 104 to each side of the treatment field prevent the applicator 102 from being positioned close to irregular surfaces on the patient 202, such as near the head and neck. This results in the final aperture 110 being further away from the patient 202 than desired, and prevents optimization of the penumbra.
Accordingly, there is a need to minimize the lateral extention of the electron applicator near the patient and minimize clearance issues to overcome the current shortcomings in the art.